Cascade ordering

ABSTRACT

A method of handling multiple served backup processes comprising creating a cascade of target storage volumes from a plurality of target storage volumes by inserting each most recent target storage volumes between a last added target storage volume and a source storage volume, the most recent target storage volumes each having a copy rate faster than the last added target storage volume, such that each added target storage volume has a faster copy rate than an immediately prior added target storage volume, and a most recent target storage volume added to the cascade of target storage volumes has a fastest copy rate; and receiving one or more instructions initiating a plurality of served backup processes from the source storage volume to a plurality of target storage volumes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/600,966, filed on Jan. 20, 2015, which is a Continuation of U.S.patent application Ser. No. 13/529,691 filed on Jun. 21, 2012, which isa Continuation of U.S. patent application Ser. No. 13/102,886, filed onMay 6, 2011, which claims priority to European Patent Application No.10163131.5, filed May 18, 2010, all of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of handling multiple backupprocesses. In one embodiment, the invention provides cascaded multipletarget flashcopy consistency group start optimisation for backgroundcopy processes.

2. Description of the Related Art

The storage of data in large organisations is of fundamental importance,both for reliability of the data and for the ability to recover data inthe event of any hardware failure. Storage area network (SAN) is anarchitecture that is used when very large amounts of data are needed tobe stored in a reliable and secure manner. This technology allowsnetworks to be created that support the attachment of remote computerstorage devices such as disk arrays to servers in such a way that, tothe operating system, the devices appear as locally attached. It iscommon in these networks to include a large amount of redundancy, bothin the data storage and in the hardware connections between theindividual components.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided amethod of handling multiple served backup processes comprising creatinga cascade of target storage volumes from a plurality of target storagevolumes by inserting each most recent target storage volumes between alast added target storage volume and a source storage volume, the mostrecent target storage volumes each having a copy rate faster than thelast added target storage volume, such that each added target storagevolume has a faster copy rate than an immediately prior added targetstorage volume, and a most recent target storage volume added to thecascade of target storage volumes has a fastest copy rate; and receivingone or more instructions initiating a plurality of served backupprocesses from the source storage volume to a plurality of targetstorage volumes.

In addition to the foregoing exemplary embodiment, various other systemand computer program product embodiments are provided and supply relatedadvantages. The foregoing summary has been provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary is not intended toidentify key features or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in determining the scopeof the claimed subject matter. The claimed subject matter is not limitedto implementations that solve any or all disadvantages noted in thebackground.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a pair of storage volumes;

FIG. 2 is a schematic diagram of a flashcopy cascade;

FIG. 3 is a schematic diagram of a plurality of storage volumes;

FIG. 4 is a schematic diagram of the construction of a cascade from thestorage volumes of FIG. 3;

FIG. 5 is a schematic diagram of the ordering of the starting of backupprocesses; and

FIG. 6 is a flowchart of a method of handling the backup processes.

DETAILED DESCRIPTION OF THE DRAWINGS

Various methods exist for creating data redundancy. For example, afunction such as a flashcopy function enables an administrator to makepoint-in-time, full volume copies of data, with the copies immediatelyavailable for read or write access. The flashcopy can be used withstandard backup tools that are available in the environment to createbackup copies on tape. A flashcopy function creates a copy of a sourcevolume on a target volume. This copy, as mentioned above, is called apoint-in-time copy. When a flashcopy operation is initiated, arelationship is created between a source volume and target volume. Thisrelationship is a “mapping” of the source volume and the target volume.This mapping allows a point-in-time copy of that source volume to becopied to the associated target volume. The relationship exists betweenthis volume pair from the time that the flashcopy operation is initiateduntil the storage unit copies all data from the source volume to thetarget volume, or the relationship is deleted.

When the data is physically copied, a background process copies tracksfrom the source volume to the target volume. The amount of time that ittakes to complete the background copy depends on various criteria, suchas the amount of data being copied, the number of background copyprocesses that are running and any other activities that are presentlyoccurring. The flashcopy function works in that the data which is beingcopied does not actually need to be copied instantaneously, it onlyneeds to be copied just prior to an update causing on overwrite of anyold data on the source volume. So, as data changes on the source volume,the original data is copied to the target volume before beingoverwritten on the source volume.

Therefore, a flashcopy is a feature supported on various storage devicesthat allows a user or an automated process to make nearly instantaneouscopies of entire logical volumes of data. A copy of a source disk ismade on a target disk. The copies are immediately available for bothread and write access. A common feature of flashcopy likeimplementations is the ability to reverse the copy. That is, to populatethe source disk of a flashcopy map with the contents of the target disk.It is also possible to use flashcopy in cascaded implementations, inwhich a target disk later becomes the source disk for a furtherflashcopy or vice versa.

A cascaded configuration of storage volumes is described in detail inU.S. Pat. No. 7,386,695. It is also possible to create multiple cascadesof storage volumes which are interlocking at a logical level. A firstcascade may comprise storage volumes A, B, C and D which are arranged ina cascade as follows: A⇄B⇄C⇄D, while at a later time a new backup of Amay be started that ultimately leads to the creation of A⇄E⇄F. Manydifferent combinations of flashcopy functions and reversed functions arepossible, potentially creating complicated multiple cascading storagevolumes.

A typical use case for multiple target flashcopy is to create a numberof application consistent snapshots of a production disk (the sourcevolume of the flashcopies). This is done using some form of aconsistency group in order to guarantee that all the copies areidentical. Both the cascaded and the “classical” implementations ofmultiple target flashcopy do not minimise the read access redirected tothe production disk effectively as part of the background copy processused to create the independent copies.

One solution to this problem, which is appropriate to both the classicaland cascaded approaches to multiple flashcopy implementations would beto perform a single read of the source disk and multiple writes to eachof the targets. However such a solution is not scalable with the numberof targets created. This solution also makes it difficult to havedifferent background copy rates for different flashcopy maps. Anothersolution appropriate to the cascaded approach is to ensure that the laststarted map completes its background copy process before the backgroundprocess of the next last started map begins. The problem with thissolution is that the length of time taken to produce an independent copyincreases with the number of copies required.

Owing to the invention, it is possible to provide system that willschedule a cascade of backup processes (such as flashcopy) that willminimise the number of reads that are required of the host disk (thesource storage volume), but without slowing down any of the actualbackup processes. The solution improves the cascaded implementation ofmultiple target flashcopy by ordering the adding to the cascade of theflashcopy maps for a single source in a consistency group inversely withrespect to the background copy rate of the map and suppressing the startof a flashcopy map's background copy process until the background copyprocess of each map added to the cascade later has been started.

The advantage of this approach is that each map can have its ownbackground copy rate, each map does not need to wait for “younger” mapsto complete the background copy process and the solution is scalablewith respect to the number of copies made. The total number of read andwrite operations required to complete the background processes for allmaps is unchanged, but the number of read operations directed to theproduction disk is reduced.

For example, with a production disk A it is possible to make copies B,C, D and E of this disk using flashcopy, by creating flashcopy maps A→B,A→C, A→D and A→E in a single consistency group with background copyrates w, x, y, z respectively. If the copy rates of the maps are suchthat w≧x≧y≧z, then when the consistency group is started, the map A→E isadded to the cascade first, because this has the lowest background copyrate, z and so on. This results in cascade A→B→C→D→E. When all maps arein the cascade, the background copy process of A→B is started first,followed by the backup process for A→C and so on. This means that thebackup process A→B copies a grain of data from volume A to volume Bfirst. Then backup process A→C will copy the data from B→C and so on. Soit can be seen that only one read of the source storage volume (theproduction disk A) is required for all four copies to be completed,because the background copy rates of “younger” maps are greater than“older” ones.

FIG. 1 illustrates the concept of a flashcopy using a storage controller8 and two storage disks 10 and 12. The disks 10 and 12 could form partof a larger array of disks, and would typically form part of anenterprise storage solution. The disks 10 and 12 could be part of astorage solution relating to a commercial website, for example. If atany time a backup needs to be made of the content of vdisk1, then aflashcopy instruction can be sent from the storage volume controller 8to that disk 10, which defines a source disk 10 (vdisk1) and also atarget disk 12 (vdisk2), which is the target of the flashcopy. Theflashcopy instruction creates a point-in-time copy of the image of thespecific vdisk which is the source disk 10.

In the example of FIG. 1, the source disk 10 of a first flashcopyinstruction is vdisk1, and the target disk 12 is vdisk2. The flashcopyinstruction starts the flashcopy process, which creates a map 14 fromthe source disk 10 to the target disk 12. This map is labelled map 1 inthe Figure. The image of vdisk1 at this specific point in time is nowavailable on vdisk2. This creates a backup of the data on vdisk1, andalso allows tests and other administration tasks to be run on the dataof vdisk1, without the attendant danger of losing any of the originaldata, as it is preserved on the original source disk.

When a flashcopy is made, it creates a link between the two disks 10 and12, as defined by the map 14. Data may now be copied across in thebackground, with the additional requirement that any access to vdisk2(as the target disk 12) will immediately cause the relevant parts of theimage of vdisk1 to be copied across, and also any access to vdisk1 whichwould result in a change to the image stored by that disk 10 will alsocause the unaltered data to be immediately copied across to the targetdisk 12, prior to the change being made. In this way, the vdisk2, to anoutside user, stores the point in time copy of vdisk1, although datawill only be physically copied across under the circumstances describedabove.

A storage volume that is the target volume of a flashcopy function canalso be the source volume of a further flashcopy function, thus creatinga cascade of storage volumes. In FIG. 2 there is shown an example of aflashcopy cascade of three storage volumes 10, 12 and 16, which arelinked by respective flashcopy maps 14. Each map 14 defines a flashcopyfunction from a source volume to a target volume. Disk B is providing abackup of disk A, and disk C is providing a backup of disk A, throughdisk B. The flashcopy functions 14 linking the different storage volumesmay have been started at different times, which create differentpoint-in-time copies of the images stored by the respective storagevolumes, or could have been started simultaneously.

In the flashcopy cascade of A→B→C, where A, B and C are the disks in thecascade, as shown in FIG. 2, and the arrows are the flashcopy maps, thendenoting (A, B) to be a flashcopy mapping from disk A to disk B, thecascade has maps (A, B) and (B, C). In this implementation of thecascade, any new data write to disk A will cause a write to disk B, asper the respective flashcopy function, which is required to maintain theimage on disk B. This writing to disk B this will cause a further cleanread of disk B followed by a write to disk C. In this way a single writeto the first storage volume 10 in the cascade can result in a number ofoperations throughout the cascade.

When a cascade is created, the new maps and new storage volumes areinserted into the cascade, not added to the end of the cascade. In thecascade shown in FIG. 2, the first backup process started would be A→C.When the backup process A→B is then started, the new target storagevolume B is effectively “inserted” between the existing source storagevolume A and the existing target storage volume C. This “insertion” ispurely a logical construction illustrating the fact that target disk Cwill receive data writes from disk B, rather than disk A. This is how acascaded implementation differs from a conventional arrangement whichwould have two independent maps from disk A.

When a cascade is formed by the simultaneous taking of multiple backupsfrom the disk A, then an ordering of the cascade is created using thedifferent copy rates of the background backup processes that have beenstarted. For example, a user may simultaneously request that threedifferent copies be made of a production disk A. The different copiesmay be taken for different purposes. A first backup may be a true backupof the data on disk A in order to guard against any failure of disk A. Asecond backup may be taken to be used to test certain processes on thedata, without the danger of losing any of the original data, while athird backup may be taken for the purpose of reading the data to tape,to create a permanent record.

The different backup processes can therefore have different copy ratesassociated with them, which can be user defined, or more likely will beinitially set by administrator at a system level and used automaticallyaccording to the purpose of the backup process being taken. For example,the backup process initiated to perform testing on the stored data willhave a higher copy rate assigned, as the backup of the data on disk A,for this purpose, needs to be completed as quickly as possible, in orderto allow the testing to start as soon as possible. Contrastingly, thebackup process simply to make a copy of the data on a different disk asa true backup of the data may have a lower copy rate assigned.

FIG. 3 illustrates a plurality of storage volumes 10 to 18 to be used tocreate a flashcopy cascade that is to be ordered with respect to threedifferent backup processes, which can be characterised by different copyrates “fast”, “slow” and “medium”. These copy rates are relative termsand define the rate at which data is copied from the source storagevolume A (the actual production disk) to the individual target storagevolumes. Although the copy rates are shown as labelling the specificstorage volumes, it should be understood that the copy rates relate tothe backup process that is copying the data to the specific storagevolume, not the actual storage volume itself. In hardware terms, thetarget storage volumes are identical.

The process of creating the cascade from the storage volumes of FIG. 3is shown in FIG. 4. Each target storage volume is added in turn to thecascade of target storage volumes that cascades from the source storagevolume, in such a way that the target storage volumes are added to thecascade in an order that is inversely proportional to the copy rate ofthe respective backup process. It can be seen in FIG. 4, that the firstaction taken is the adding of target storage volume D is first added tothe cascade, as this storage volume has the slowest copy rate of thebackup process that is associated with that specific storage volume.

The second action is the addition of the target storage volume C, whichhas “medium” copy rate for the backup process that is associated withthat specific storage volume C. The cascade now contains three storagevolumes, the original production disk A (which is the source storagevolume) and the two target storage volumes C and D. Note that the natureof the cascade addition rule is that the last added storage volume isplaced closest to the source storage volume A, so that the addition ofthe target storage volume C means that this disk is inserted betweenvolumes A and D, in the logical arrangement of the storage volumes.

Finally, the third target storage volume, disk B, is added to thecascade. This storage volume has the fastest copy rate for the backupprocess that is associated with the specific disk B, so this targetstorage volume is added to the cascade last, as the volumes are added inan order that is inversely proportional to their associated copy rate.As per the algorithm for adding disks to a cascade, the last addedstorage volume, disk B, is placed closest to the source storage volumeA. The storage volume B is therefore inserted between the productiondisk A and the target storage volume C. This action completes thecascade.

Once the cascade has been created, then the backup processes that definethe maps between the storage volumes need to be started. The storagevolume controller 8 will start each backup process in turn, the backupprocesses being started in an order from the most recent target storagevolume added to cascade to the first target storage volume added tocascade. As can be seen in FIG. 5, this means that the backup processthat is started first is the process that is associated with the targetstorage volume B, as this was the last storage volume to be added to thecascade. As can be seen in the Figure, this backup process is startedfirst.

The remaining backup processes from the production disk A, which formthe cascade, are then started in turn. After the backup process forstorage volume B has been started then the backup process for volume Cis started, followed by the start of the backup process for volume D. Inthis way, the backup processes are started one after another until allhave been started. The order in which the processes are started is theopposite of the order by which the different storage volumes were addedto the cascade. Faster backup processes are started before slower backupprocesses. Once the cascade has been ordered and the backup processesstarted, then the flashcopy cascade will run as normal.

The methodology described above for the addition of the volumes to thecascade and the starting of the backup processes must also be able todeal with the situation when two (or more) backup processes that arescheduled to start together actually have the same copy rate. This caneasily happen if, for example, a user wishes to perform two differenttesting scenarios on the dataset of production disk A and willcorrespondingly schedule two backup processes from disk A to respectivetarget storage volumes. The storage volume controller 8 can make anarbitrary decision on which to add to the cascade first, as this isimmaterial to the overall performance, but must still start the backupprocesses in the inverse order to which the volumes were added to thecascade.

FIG. 6 is a flowchart summarizing the operation of the storage volumecontroller 8 in performing the flashcopy scheduling function. The firststep S1 comprises receiving one or more instructions initiating aplurality of backup processes from a single source storage volume to aplurality of target storage volumes. All of the backup processes may betriggered by a single instruction from a user, or multiple instructionsmay be sent by the user to initiate the taking of the backups from theproduction disk. Essentially multiple copies are to be taken from thesame source storage volume with multiple independent backup processes.The storage volume controller 8 is responsible for the scheduling ofthese tasks.

The second step S2 performed by the storage volume controller 8comprises the step of adding each target storage volume to a cascade oftarget storage volumes from the source storage volume, the targetstorage volumes being added to the cascade in an order that is inverselyproportional to the copy rate of the respective backup process. Thisstep is described above in detail with reference to the example of FIG.4. Essentially the target storage volumes are added in order to thecascade, with the volume associated with the slowest copy rate beingadded to the cascade. After each individual addition, then the nextvolume associated with the next slowest copy rate is added to thecascade.

The final step of the method is step S3 in which each backup process isstarted in turn, the backup processes being started in an order from themost recent target storage volume added to cascade to the first targetstorage volume added to cascade. In a preferred embodiment, this step ofstarting each backup process in turn comprises suppressing the start ofeach backup process until all of the backup processes for target storagevolumes added later to the cascade have been started. This step isdescribed above in detail with reference to the example of FIG. 5. Thebackup processes are each started in turn, in the opposite order towhich the corresponding storage volumes were added to the cascade.

The scheduling of the cascade of backup processes described aboveminimizes the number of reads that are required of the production disk,but without slowing down any of the actual backup processes. Thissolution provides an improvement in the cascaded implementation ofmultiple target flashcopy. This is achieved by ordering the adding tothe cascade of the flashcopy maps for a single source in a consistencygroup inversely with respect to the background copy rate of the map andsuppressing the start of a flashcopy map's background copy process untilthe background copy process of each map added to the cascade later hasbeen started. The main advantage of this approach is that eachbackground process will have its own background copy rate and does notneed to wait for other background copy processes to complete. The totalnumber of read and write operations required to complete the backgroundprocesses for all backup processes is unchanged, but the number of readoperations directed to the production disk is reduced.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While one or more embodiments of the present invention have beenillustrated in detail, the skilled artisan will appreciate thatmodifications and adaptations to those embodiments may be made withoutdeparting from the scope of the present invention as set forth in thefollowing claims.

1. A method of handling multiple served backup processes comprising:creating a cascade of target storage volumes from a plurality of targetstorage volumes by inserting each most recent target storage volumesbetween a last added target storage volume and a source storage volume,the most recent target storage volumes each having a copy rate fasterthan the last added target storage volume, such that each added targetstorage volume has a faster copy rate than an immediately prior addedtarget storage volume, and a most recent target storage volume added tothe cascade of target storage volumes has a fastest copy rate; andreceiving one or more instructions initiating a plurality of servedbackup processes from the source storage volume to a plurality of targetstorage volumes.
 2. The method of claim 1, further including startingeach served backup process in turn, the served backup processes startedin an order from a most recent target storage volume added to thecascade of target storage volumes to the first target storage volumeadded to the cascade of target storage volumes.
 3. A method according toclaim 1, wherein each served backup process comprises a flashcopyfunction creating a point-in-time copy of a first source storage volumeon a corresponding target storage volume.
 4. A method according to claim3, further comprising detecting that two or more served backup processeshave a same copy rate and, when adding each target storage volume to thecascade of target storage volumes, making an arbitrary decision on whichof the target storage volumes with served backup processes having thesame copy rate should be added to the cascade of target storage volumesfirst.
 5. A method according to claim 2, wherein the step of startingeach served backup process in turn comprises suppressing a start of eachserved backup process until all served backup processes for targetstorage volumes subsequently added to the cascade have been started. 6.A method according to claim 5, further comprising detecting that two ormore served backup processes have a same copy rate and, when adding eachtarget storage volume to the cascade of target storage volumes, makingan arbitrary decision on which of the target storage volumes with servedbackup processes having the same copy rate should be added to thecascade of target storage volumes first.
 7. A method according to claim2, wherein the step of starting each served backup process in turncomprises suppressing a start of each served backup process until allserved backup processes for target storage volumes subsequently added tothe cascade have been started.
 8. A method according to claim 1, furthercomprising detecting that two or more served backup processes have asame copy rate and, when adding each target storage volume to thecascade of target storage volumes, making an arbitrary decision on whichof the target storage volumes with served backup processes having thesame copy rate should be added to the cascade of target storage volumesfirst.